1. The Field of the Invention
This invention relates to the detection of defective Floppy Diskette Controllers ("FDCs") where an undetected data error causes data corruption and, more particularly, to novel systems and methods implemented as a software-only detection mechanism which eliminates the need for visual inspection or identification of the FDCs.
2. The Background Art
Computers are now used to perform functions and maintain data that is critical to many organizations. Businesses use computers to maintain essential financial and other business data. Computers are also used by government to monitor, regulate, and even activate, national defense systems. Maintaining the integrity of the stored data is essential to the proper functioning of these computer systems, and data corruption can have serious (even life threatening) consequences.
Most computer systems include diskette drives for storing and retrieving data on floppy diskettes. For example, an employee of a large financial institution may have a personal computer that is attached to the main system. In order to avoid processing delays on the mainframe, the employee may routinely transfer data files from a host system to a local personal computer and then back again, temporarily storing data on a local floppy diskette. Similarly, an employee with a personal computer at home may occasionally decide to take work home, transporting data away from and back to the office on a floppy diskette.
Data transfer to and from a floppy diskette is controlled by a device called a Floppy Diskette Controller ("FDC"). The FDC is responsible for interfacing the computer's Central Processing Unit ("CPU") with the physical diskette drive. Significantly, since the diskette is spinning, it is necessary for the FDC to provide data to the diskette drive at a specified data rate. Otherwise, the data will be written to a wrong location on the diskette.
The design of an FDC accounts for situations occurring when a data rate is not adequate to support a rotating diskette. Whenever this situation occurs, the FDC aborts the write operation and signals to the CPU that a data underrun condition has occurred.
Unfortunately, however, it has been found that a design flaw in many FDCs makes impossible the detection of certain data underrun conditions. This flaw has, for example, been found in the NEC 765, INTEL 8272 and compatible Floppy Diskette Controllers. Specifically, data loss and/or data corruption may routinely occur during data transfers to or from diskettes (or even tape drives and other media attached via the FDC), whenever the last data byte of a sector being transferred is delayed for more than a few microseconds. Furthermore, if the last byte of a sector write operation is delayed too long then the next (physically adjacent:) sector of the diskette will be destroyed as well.
For example, it has been found that these faulty FDCs cannot detect a data underrun on the last byte of a diskette read or write operation. Consequently, if the FDC is preempted or otherwise suspended during a data transfer to the diskette (thereby delaying the transfer), and an underrun occurs on the last byte of a sector, the following occur: (1) the underrun flag does not get set, (2) the last byte written to the diskette is made equal to the previous byte written, and (3) a successful Cyclic Redundancy Check ("CRC") is generated on the improperly altered data. The result is that incorrect data is written to the diskette and validated by the FDC.
Conditions under which this problem may occur have been identified in connection with the instant invention by identifying conditions that can delay data transfer to or from the diskette drive. In general, this requires that the computer system be engaged in "multi-tasking" operation or in overlapped input/output ("I/O") operation. Multi-tasking is the ability of a computer operating system to simulate the concurrent execution of multiple tasks.
Importantly, concurrent execution is only "simulated" because only one CPU exists in a typical personal computer. One CPU can only process one task at a time. Therefore, a system interrupt is used to rapidly switch between the multiple tasks, giving the overall appearance of concurrent execution.
MS-DOS and PC-DOS, for example, are single-task operating systems. Therefore, one could argue that the problem described above would not occur. However, a number of standard MS-DOS and PC-DOS operating environments simulate multi-tasking and are susceptible to the problem.
In connection with the instant invention, for example, the following environments have been found to be prime candidates for data loss and/or data corruption due to defective FDCs: local area networks, 327x host connections, high density diskettes, control print screen operations, terminate and stay resident ("TSR") programs. The problem also occurs as a result of virtually any interrupt service routine. Thus, unless MS-DOS and PC-DOS operating systems disable all interrupts during diskette transfers, they are also highly susceptible to data loss and/or corruption.
The UNIX operating system is a multi-tasking operating system. It has been found, in connection with the instant invention, how to create a situation that can cause the problem within UNIX. One example is to begin a large transfer to the diskette and place that transfer task in the background. After the transfer has begun then begin to process the contents of a very large file in a way that requires the use a Direct Memory Access ("DMA") channel of a higher-priority than that of the floppy diskette controller's DMA channel. These might include, for example, video updates, multi-media activity, etc. Video access forces the video buffer memory refresh logic on DMA channel 1, along with the video memory access, which preempts the FDC operations from occurring on DMA channel 2 (which is lower priority than DMA channel 1).
This type of example creates an overlapped I/O environment and can force the FDC into an undetectable error condition. More rigorous examples include a concurrent transfer of data to or from a network or tape drive using a high priority DMA channel while the diskette transfer is active. Clearly, the number of possible error producing examples is infinite, yet each is highly probable in this environment.
For all practical purposes the OS/2 and newer Windows operating systems can be regarded as UNIX derivatives. They suffer from the same problems that UNIX does. Two significant differences exist between these operating systems and UNIX.
First, they both semaphore video updates with diskette operations tending to avoid forcing the FDC problem to occur. However, any direct access to the video buffer, i:n either real or protected mode, during a diskette transfer will bypass this feature and result in the same faulty condition as UNIX.
Second, OS/2 incorporates a unique command that tends to avoid the FDC problem by reading back every sector that is written to the floppy diskette in order to verify that the operation completed successfully. This command is an extension to the MODE command (MODE DSKT VER=ON). With these changes, data loss and/or data corruption should occur less frequently than otherwise. However, the FDC problem may still destroy data that is not related to the current sector operation.
A host of other operating systems are susceptible to the FDC problem just as DOS, Windows, Windows 95, Windows NT, OS/2, and UNIX. However, these systems may not have an installed base as large as DOS, Windows, OS/2 or UNIX, and may, therefore, receive less motivation to address the problem. Significantly, as long as the operating systems utilize the FDC and service system interrupts, the problem can manifest itself. This can occur in computer systems that use virtually any operating system.
Some in the computer industry have suggested that data corruption by the FDC is extremely rare and difficult to reproduce. This is similar to the argument presented during the highly publicized 1994 defective INTEL Pentium scenario. Error rate frequencies for the defective Pentium ranged from microseconds to tens-of-thousands of years! The FDC problem is often very difficult to detect during normal operation because of its random characteristics. The only way to visibly detect this problem is to have the FDC corrupt data that is critical to the operation at hand. However, many locations on the diskette may be corrupted, yet not accessed. In connection with the instant invention, the FDC problem has been routinely reproduced and may be more common than heretofore believed.
Computer users may, in fact, experience this problem frequently and not even know about it. After formatting a diskette, for example, the system may inform the user that the diskette is bad, although the user finds that if the operation is performed again on the same diskette everything is fine. Similarly, a copied file may be unusable, and the computer user concludes that he or she just did something wrong. For many in this high-tech world, it is very difficult to believe that the machine is in error and not themselves. It remains typical, however, that full diskette back-ups are seldom restored, that all instructions in programs are seldom, if ever, executed, that diskette files seldom utilize all of the allocated space, and that less complex systems are less likely to exhibit the problem.
Additionally, the first of these faulty FDCs was shipped in the late 1970's. The devices were primarily used at that time in special-purpose operations in which the FDC problem would not normally be manifest. Today, on the other hand, the FDCs are incorporated into general-purpose computer systems that are capable of concurrent operation (multi-tasking or overlapped I/O). Thus, it is within today's environments that the problem is most likely to occur by having another operation delay a data transfer to a diskette. The more complex a computer system, the more likely it is that one activity will delay another, thereby creating an FDC error condition.
In short, the potential for data loss and/or data corruption is present in all computer systems that utilize the defective version of this type of FDC, presently estimated at about 50 million personal computers. The design flaw in the FDC causes data corruption to occur and manifest itself in the same manner as a destructive computer virus. Furthermore, because of its nature, this problem has the potential of rendering even secure databases absolutely useless.
Various conventional ways of addressing the FDC problem, such as a hardware recall, have significant associated costs, risks and/or disadvantages. In addition to a solution to the FDC problem, an apparatus and method are needed to accurately, rapidly, reliably, and correctly, identify any defective FDC. The identification of defective FDCs is the first step in attempting to solve the problem of defective FDCs. A solution method and apparatus for repairing a defective FDC are disclosed in U.S. Pat. No. 5,379,414 incorporated herein by reference.